A. Technical Field
The present invention relates to a production process of a water-absorbent resin, more particularly, a production process of a water-absorbent resin by polymerizing statically an aqueous solution including a hydrophilic monomer.
B. Background Art
In recent years, in the fields of sanitary materials and so on such as paper diapers, physiological napkins and so-called incontinent pads, a water-absorbent resin is widely used as the object of absorbing body fluids.
As to the above water-absorbent resin, the following materials are, for example, known: crosslinked products of partially neutralized polyacrylic acid, hydrolysis products of starch-acrylonitrile graft polymer, neutralized products of starch-acrylic acid graft polymer, saponified products of vinyl acetate-acrylic acid ester copolymer, hydrolysis products of acrylonitrile copolymer or acrylamide copolymer or their crosslinked products, crosslinked products of cationic monomer.
As a production process of these water-absorbent resins, a process of carrying out an aqueous solution polymerization while stirring an aqueous solution including a hydrophilic monomer of which the main component is acrylic acid or its salt is generally employed.
As for the stirring polymerization, because the polymerization can be carried out while cutting to little pieces a hydrogel polymer generated with the progress of the polymerization, this is excellent in that polymerization in which the polymerization peak temperature is controlled to some extent can be carried out by removing the polymerization heat with a relatively compact apparatus. However, because molecular chains are cut due to the shear force of the stirring, there are such problems that the molecular weight is hardly raised and that the network of the crosslinking structure is apt to fall into disorder. In comparison with this process, by a process of polymerizing statically an aqueous monomer solution without stirring, as is suggested in JP-A-62-156102, JP-A-01-126310, JP-A-03-174414, JP-A-04-175319, JP-A-04-236203 and so on, the water-absorbent resin is obtained without problems mentioned above.
However, in the conventional static aqueous solution polymerization, because the polymerization temperature cannot be controlled, the maximum attained temperature of the polymerization system exceeds 110.degree. C. Therefore, the water-soluble content, which is undesirable for the water-absorbent resin, is increased, so only the water-absorbent resin of low physical properties can be obtained. Or, to control the maximum attained temperature, it is necessary to lower the concentration of the aqueous monomer solution, so it is unavoidable to employ production processes of low productivity.